1. Field of the Invention
The present invention relates to an electro-resistance element, the resistance value of which changes by applying a voltage or current, and to an electro-resistance memory using the same.
2. Related Background Art
Memory elements are used in a wide variety of fields as essential electronic components that are important to support today's information-driven society. In recent years, as portable information terminals have become increasingly popular, demands for miniaturization of memory elements have been increasing, and non-volatile memory elements are not exceptions. As the scale of device miniaturization is approaching the nanometer range, however, a decrease in charge capacity C per information unit, i.e. bit, has become a problem with conventional charge storage type memory elements, typically DRAMs: Dynamic Random Access Memories. Although various improvements have been attempted to obviate this problem, there is a concern over the future technological limit.
As a memory element that is less prone to the adverse effects of miniaturization, a non-volatile memory element, i.e. an electro-resistance memory element, that records information by changes in electric resistance value R, not by changes in charge capacity C, has attracted attention. As this sort of electro-resistance memory element, an electro-resistance element that changes its electric resistance value R by applying a predetermined voltage or current is under development.
A typical electro-resistance element has a structure in which its electro-resistance layer is sandwiched by a pair of electrodes, and its electric resistance value R is changed by applying a predetermined voltage or current between the pair of electrodes. Such a change in the electric resistance value R is based on the state change of the electro-resistance layer by applying the predetermined voltage or current, and generally is known as a CER effect: a Colossal Electro-Resistance effect. A CER effect is different in principle from a so-called MR effect: a Magneto-Resistance effect, which is also a phenomenon in which an electric resistance value R of an element is changed.
An MR effect is a phenomenon in which an electric resistance value between a pair of magnetic materials sandwiching a non-magnetic material is changed when a direction of magnetization of one of the magnetic materials changes from parallel to antiparallel, or from antiparallel to parallel, to that of the other magnetic material. Currently, an MRAM: a Magneto-resistance Random Access Memory, using an element showing an MR effect, i.e. an MR element, is under development. However, as a component called a demagnetizing field in the magnetic material increases with miniaturization of the material, a magnetic field required to reverse a direction of magnetization becomes larger; thus, it is difficult to miniaturize further than a certain extent by using an MR element. As a CER effect does not have such a “size restriction” and it gains generally larger electric resistance change compared to an MR effect, an electro-resistance element is expected highly as the next generation non-volatile memory implementing even further miniaturization, such as a ReRAM: an electro-Resistance Random Access Memory.
As this sort of electro-resistance element, U.S. Pat. No. 6,204,139 and JP 2002-537627A disclose elements including perovskite oxides in electro-resistance layers. The former publication mentions the oxide with Pr0.7Ca0.3MnO3 (PCMO), and the latter uses Cr-doped BaSrTiO3 (BSTCO). JP 2004-363604A discloses an element including a metallic oxide, such as NiO, in an electro-resistance layer. Every element disclosed in the three publications above has an electro-resistance layer using an iron oxide.
In order to construct a memory cell array using electro-resistance elements, the element needs to be combined with a semiconductor element for selecting the element on recording and reading information, such as a transistor or a diode. However, crystallization of perovskite oxides, such as PCMO and BSTCO, generally requires a high temperature of approximately 650° C. to 850° C., which leads to an issue of affinity with semiconductor manufacturing processes. In particular, in order to achieve element miniaturization, it is desired that the manufacturing process temperature for the electro-resistance element be set lower than the range above, for example 400° C. or lower.
When an electro-resistance element is combined with a semiconductor element, a high-temperature heat treatment, typically at about 400° C. to 500° C., under a hydrogen-containing atmosphere is necessary for the purpose of improving switching characteristics of the semiconductor element, such as reducing wiring resistance. The heat treatment, however, is prone to deteriorate resistance change characteristics of an element using a perovskite oxide, such as PCMO.
On the other hand, the element disclosed in JP 2004-363604A has its electro-resistance layer made of a metallic oxide, such as NiO, and it is considered that the element can be formed at a lower temperature compared to perovskite oxides. It further is expected to lower the temperature during the manufacturing processes of the element. The resistance change characteristics of the element disclosed in JP 2004-363604A are, however, prone to be deteriorated by a heat treatment under a hydrogen-containing atmosphere similar to that of the elements disclosed in U.S. Pat. No. 6,204,139 and JP 2002-537627A. This is supposedly because the electro-resistance layer of each element disclosed in the three publications is made of an oxide and the oxide, and deoxidization by exposure to hydrogen during the heat treatment causes a structural defect, such as extreme oxygen deficiency, of the electro-resistance layer.
It is an object of the present invention to provide an electro-resistance element that has a different configuration from those of the conventional elements with their electro-resistance layers made of an oxide and is excellent in its affinity with semiconductor manufacturing processes as well as its heat treatment stability under a hydrogen-containing atmosphere.
Another object of the present invention is to provide an electro-resistance memory using the same.